The present invention relates to a semiconductor device having a gate turn-off (GTO) thyristor.
A GTO thyristor is a kind of reverse blocking triode thyristor, which is made conductive in response to a trigger pulse and self-holds this conduction state. Generally, a reverse blocking triode thyristor cannot be turned off without a commutation circuit functioning to reverse the current flowing through the thyristor. However, this GTO thyristor has the feature such that it can become nonconductive in response to a trigger pulse of the opposite polarity. Therefore, this GTO thyristor can be connected between a DC power source and a load as a switching element.
FIG. 1 shows a structure of a conventional GTO thyristor. This GTO thyristor includes an anode emitter 10P and a cathode base 12P which are formed of a p-type semiconductor, and an anode base 14N and a plurality of (for example, three) cathode emitters 16N-1 to 16N-3 which are formed of an n-type semiconductor. The anode emitter 10P is connected to an anode terminal A through an anode electrode 18. The cathode emitters 16N-1 to 16N-3 are respectively connected to a terminal K through cathode electrodes 20-1 to 20-3. The DC power source and load (not shown) can be connected in series to the GTO thyristor through the terminals A and K. The cathode base 12P is connected to a terminal G through a gate electrode 22 which is formed on the cathode base 12P so as to surround the cathode emitters 16N-1 to 16N-3 on a plane.
In the GTO thyristor of FIG. 1, when the potential of the terminal G is set higher than that of terminal K to apply a forward bias voltage to the p-n junctions between the cathode base 12P and each of the cathode emitters 16N-1 to 16N-3, a current path is formed between the terminals A and K. On the other hand, the potential of the terminal G is set lower than that of the terminal K to supply a reverse bias voltage, this current path is extinguished. In addition, the reason why a plurality of cathode emitters 16N-1 to 16N-3 surrounded by the gate electrode 22 are provided for the GTO thyristor is to allow a current to uniformly flow in the semiconductor elements among the cathode electrodes 16N-1 to 16N-3 and the anode electrode 18. Otherwise, there is the fear that some semiconductor elements will be destroyed due to a centralized current to be caused at the time of a sudden current change.
On the other hand, a very large trigger current is needed to turn off the GTO thyristor with such a structure. For example, there is a large-sized GTO thyristor which is turned on by a trigger current of at least 1A and which can present a good turn-on characteristic by a trigger current of about 10A; however, a trigger current of at least several hundreds of A is necessary to turn off this GTO thyristor.
On one hand, FIG. 2 shows a GTO thyristor in which a current supply section for supplying a driving current from the anode electrode 18 to the gate electrode 22 was added to the GTO thyristor shown in FIG. 1. This GTO thyristor has a second cathode emitter 24N and a second gate electrode 26 which is formed of an n-type semiconductor. The cathode emitter 24N and gate electrode 26 are both formed on the cathode base 12P. The gate electrode 22 is formed in contact with the cathode emitter 24N and the terminal G is connected to the gate electrode 26. The gate electrode 22 is connected to the gate electrode 26 through a diode 28 for supplying a reverse bias voltage, corresponding to the voltage drop, between the cathode base 12P and the cathode emitter 24N to attain the turn-off condition.
The GTO thyristor with such a structure can be rendered conductive by a trigger current which is about one tenth of that in case of the thyristor shown in FIG. 1. However, in this GTO thyristor, since the gate electrode 22 partially short-circuits the cathode base 12P and cathode emitter 24N, a reverse bias voltage is not sufficiently applied to the p-n junction between the cathode base 12P and cathode emitter 24N when the potential of the terminal G is set lower than that of terminal K to effect turn-off operation. Due to this, it takes a long time until the current which flows through the p-n junction is completely cut off, which results in a loss of electric power. In addition, if a large current is flowing between the terminals A and K, there may be a case where this p-n junction will be damaged due to the current therethrough at the time of turn-off.
FIG. 3 illustrates a thyristor with a metal oxide silicon (MOS) gate structure. This thyristor has a p-type layer 32P to which an anode electrode 30 is connected, an n-type layer 34N which is formed on this layer 32P, a p-type layer 36P which is formed in the surface area of the layer 34N, and an n-type layer 38N which is formed in the surface area of the layer 36P. A cathode electrode 40 is formed on the layers 36P and 38N. The gate electrode 42 is formed on the layers 34N, 36P and 38N through an insulation layer 44.
When the potential of the terminal G is set lower than that of the terminal K, a current channel is formed in the portion of the layer 36P located immediately under the insulation layer 44, so that a current flows from the anode electrode 30 to the cathode electrode 40 through the channel. When this current is larger than a predetermined value (i.e., breakover current), an electrical barrier disappears between the layers 34N and 36P, thereby allowing a current to flow from the layer 36P to the layer 38N irrespective of the presence and absence of the channel. In this case, once this thyristor has been made conductive, it is impossible to control through the terminals G and K so as to make it nonconductive. However, the above-mentioned thyristor has the capability of being made conductive by only a current on the order of .mu.A for charging the capacitance between the gate electrode 42 and the layer 36P.
The thyristor shown in FIG. 4 is substantially identical to that in FIG. 3 in principle, although a p-type layer 46 to which an anode electrode 48 has been connected is formed in the surface area of the layer 34N.